1. Technical Field
The present invention relates to a method of precisely controlling the molecular weight and chain shape of a polymer using a compound having an alcohol or carboxylic acid functional group as a chain transfer agent when carrying out a process of preparing an alternating copolymer of carbon dioxide and epoxide using a catalyst comprising a trivalent metal complex synthesized from a quaternary ammonium salt-containing Salen type ligand, and to a low-molecular-weight poly(alkylene carbonate) compound prepared thereby.
2. Description of the Related Art
Poly(alkylene carbonate) is a polymer which is easily biodegradable, and is useful as an exemplary example of a packaging or coating material. Preparation of poly(alkylene carbonate) from an epoxide compound and carbon dioxide is very environmentally friendly because phosgene which is poisonous is not used and carbon dioxide may be inexpensively obtained.
Many researchers have developed various types of catalysts in order to produce poly(alkylene carbonate) from an epoxide compound and carbon dioxide since 1960. The present inventors have recently disclosed a catalyst having high activity and high selectivity synthesized from a quaternary ammonium salt-containing Salen [H2Salen=N,N′-bis(3,5-dialkylsalicylidene)-1,2-ethylenediamine]type ligand [Bun-Yeoul Lee, Korean Patent No. 10-0853358 (issue date: 2008 Aug. 13); Bun-Yeoul Lee, Sujith S, Eun-Kyung Noh, Jae-Ki Min, Korean Patent Application No. 10-2008-0015454 (filing date: Feb. 20, 2008); Bun-Yeoul Lee, Sujith S, Eun-Kyung Noh, Jae-Ki Min, PCT/KR2008/002453 (filing date: 2008 Apr. 30); Eun-Kyung Noh, Sung-Jae Na, Sujith S, Sang-Wook Kim, and Bun-Yeoul Lee J. Am. Chem. Soc. 2007, 129, 8082-8083 (2007 Jul. 4); Sujith S, Jae-Ki Min, Jong-Eon Seong, Sung-Jae Na, and Bun-Yeoul Lee, Angew. Chem. Int. Ed., 2008, 47, 7306-7309 (2008 Sep. 9)]. The catalyst disclosed by the present inventors may be highly active and highly selective and enables the preparation of a copolymer having high molecular weight and polymerization to be carried out even at high temperature and thus may be applied to commercial processes. Furthermore, this catalyst is advantageous because a quaternary ammonium salt is contained in the ligand, and thus the catalyst may be easily separated from a copolymer resulting from copolymerization of carbon dioxide and epoxide, and re-used.
Also, the present inventors have carefully examined a catalyst having higher activity and higher selectivity among the catalyst group of the above patent and thus have proved that such a catalyst has a peculiar structure in which a nitrogen atom of the Salen ligand is not coordinated but only an oxygen atom is coordinated to a metal, which was not known to date (see Structure 1 below, Sung-Jae Na, Sujith S, Anish Cyriac, Bo-Eun Kim, Jina Yoo, Youn K. Kang, Su-Jung Han, Chongmok Lee, and Bun-Yeoul Lee “Elucidation of the Structure of A Highly Active Catalytic System for CO2/Epoxide Copolymerization: A Salen-Cobaltate Complex of An Unusual Binding Mode” Inorg. Chem. 2009, 48, 10455-10465).

Furthermore, a method of easily synthesizing the ligand of the compound of Structure 1 has been developed (Min, J.; Seong, J. E.; Na, S. J.; Cyriac, A.; Lee, B. Y. Bull. Korean Chem. Soc. 2009, 30, 745-748).
High-molecular-weight poly(alkylene carbonate) may be economically prepared using the compound of Structure 1 as a highly active catalyst. However, because poly(alkylene carbonate) has a low glass transition temperature (which is 40° C. in the case of poly(alkylene carbonate) prepared from propylene oxide and carbon dioxide) and has insufficient mechanical strength, predetermined limitations are imposed on the applications that can be developed therewith.
With the goal of overcoming the limitations of poly(alkylene carbonate), methods of preparing poly(alkylene carbonate)polyol having low molecular weight and a plurality of —OH terminal groups and preparing polyurethane therefrom have been developed. Polyurethane is a polymer obtained by reacting a compound having an —OH group with a compound having an isocyanate (—NCO) group thus forming a urethane bond (—NHC(O)O—). A variety of compounds having an —NCO group and compounds having an —OH group are being developed, and thermoplastic or thermosetting plastics or elastomeric polyurethanes having various properties have been developed and used. Polyurethane is prepared and used in an amount of about twelve million tons all over the world in 2007, and the amount thereof is increased by 5% per year and the applications thereof have become wide. Examples of the compound having an —OH group include diols and polyester diols having —OH terminal groups, which have thousands of molecular weights obtained by ring-opening polymerization of ethylene oxide or propylene oxide. Attempts have been made to prepare poly(alkylene carbonate)diol or polyol to be used instead of poly(alkylene oxide)diol or polyester diol in order to attain polyurethane (W. Kuran, in: Polymeric Materials Encyclopedia, J. C. Salamone, Ed. CRC Press, Inc., Boca Raton 1996, Vol. 9, p. 6623; Polymer, 1992, vol 33, 1384). Polyurethane prepared from poly(alkylene carbonate)polyol is known to have higher hydrolyzability compared to urethane prepared from polyester polyol (EP 302712; U.S. Pat. No. 5,863,627), and is also reported to have greater antistatic effects (U.S. Pat. No. 4,931,486). Furthermore, thrombus coagulation resistance is reported to be high (WO 9857671).
EP 302712 (priority filing date: 1987 Aug. 4) and EP 311278 (priority filing date: 1987 Oct. 6) disclose copolycarbonate diol prepared by polycondensing diethylcarbonate (EtOC(O)OEt) and 1,6-hexanediol or 1,6-petandediol, and preparation of polyurethane therefrom. Also, U.S. Pat. No. 5,171,830 (filing date: 1991 Aug. 16) discloses a method of synthesizing poly(alkylene carbonate) by polycondensing dialkyl carbonate (ROC(O)OR) and alpha,omega-alkanediol having 4 or more carbons in the presence of a base catalyst and preparation of a urethane resin using the same.
EP 798328A2 (priority filing date: 1996 Mar. 28) discloses synthesis of polycarbonate copolyether diol using polycondensation of polyether diol and dimethylcarboante (MeOC(O)OMe).
Also, synthesis of poly(alkylene carbonate)macrodiol using polycondensation of various diols and ethylene carbonate and preparation of polyurethane using the same are disclosed in the Journal of Applied Polymer Science, 1998, 69, 1621-1633 and the Journal of Applied Polymer Science, 1989, 37, 1491-1511.
However, such poly(alkylene carbonate)polyol is not prepared using copolymerization of carbon dioxide and epoxide and also has a structure different from that of a copolymer of carbon dioxide and epoxide. Specifically, in order to prepare poly(alkylene carbonate) using polycondensation of ethylene carbonate or dialkyl carbonate, diol spaced apart by 3 or more carbons should be used. Namely, a carbonate bond has a structure connected by 3 or more carbons. Poly(alkylene carbonate) resulting from copolymerization of carbon dioxide and epoxide is configured such that a carbonate bond is connected by 2 carbons.
U.S. Pat. No. 4,686,276 (filing date: 1985 Dec. 30) discloses a method of synthesizing poly(ethylene carbonate)diol by copolymerizing carbon dioxide and ethylene oxide in the presence or absence of ethylene carbonate using a catalyst comprising an alkaline compound and a tin compound and an initiator comprising a diol compound. Also, U.S. Pat. No. 4,528,364 (filing date: 1984 Apr. 19) discloses a method of removing a catalyst from the prepared polymer compound. As such, the produced polymer has a carbon dioxide content of less than 30% and is not a complete alternating copolymer. In addition, preparation of polyurethane using poly(ethylene carbonate)diol which was prepared and purified by the above method is disclosed in the Journal of Applied Polymer Science, 1990, 41, 487-507
EP 0222453 (filing date: 1986 Jun. 11) discloses a method of synthesizing polyol by copolymerizing carbon dioxide and epoxide using an organic material having a reactive proton as a chain transfer agent in the presence of a double metal cyanide compound as a catalyst. However, the obtained polyol has a carbon dioxide content of 5˜13 mol % and is not a pure poly(alkylene carbonate) compound based on complete alternating copolymerization of carbon dioxide and epoxide.
CN 1060299A (filing date: 1994 Sep. 19), which is published later, discloses a method of preparing polyol by copolymerizing carbon dioxide and epoxide using an organic material having 1˜10 reactive protons as a chain transfer agent in the presence of a polymer-supported bimetallic catalyst. However, the polyol thus obtained has a carbon dioxide content of 37˜40 mol % and is thus not a pure poly(alkylene carbonate) compound based on complete alternating copolymerization of carbon dioxide and epoxide.
There are many reports related to synthesis of low-molecular-weight poly(alkylene carbonate) using alternating copolymerization of carbon dioxide and epoxide. The copolymerization of carbon dioxide and epoxide is a living or immortal polymerization in which a polymer chain is grown from the chain initiator of the catalyst. For this reason, when the activity of the catalyst is low, a low-molecular-weight polymer is obtained. Most catalysts, except for the catalyst developed by the present inventors as mentioned above, have low activity, resulting in poly(alkylene carbonate) having a low molecular weight of 50,000 or less. However, the preparation of low-molecular-weight poly(alkylene carbonate) using a catalyst having low activity requires a large amount of catalyst, negating monetary benefits. Furthermore, because the polymer chain grows in one direction from the chain initiator of the catalyst, one end of the chain has the chain initiator of the catalyst and the other end thereof has —OH. Typically, the chain initiator included in the catalyst is alkoxy, aryloxy, carboxyl or halide. When the polymer chain has such a shape, it cannot be used to prepare polyurethane even when having low molecular weight.